I just switched from forehand to backhand a few months ago, and the first 2 weeks or so, my right oblique would be killing me the day after a round. It just seemed to slowly get a little better, and now I never feel it. So yes it is normal until your muscles man up!

Be careful, I tore my right external oblique and cracked few ribs a few years ago. I wasn't using my hips and posture properly. Took 3 months to recover, but was throwing further with ease after learning to use hips and keep better balance/posture.

You could also try to strengthen your core by doing exercises 3 times a week. Here is an oblique V-ups exercise from the Ab-ripper X portion of P90x and you will feel it if you do it correctly . Once you get healed, this may help prevent future injuries to that area.